Data from: Environmental and evolutionary effects on horn growth of male bighorn sheep

The development of male secondary sexual characters such as antlers or horns has substantial biological and socio-economic importance because in many species these traits affect male fitness positively through sexual selection and negatively through trophy hunting. Both environmental conditions and selective hunting can affect horn growth but their relative importance remains unexplored. We first examined how a large-scale climate index, the Pacific Decadal Oscillation (PDO), local weather and population density influenced both absolute and relative annual horn growth from birth to 3 years of male bighorn sheep (Ovis canadensis) over 42 years. We then examined the relative influence of environmental conditions and evolution mainly driven by trophy hunting on male horn length at 3 years of age. Horn growth was positively influenced by low population density and warm spring temperature, suggesting that ongoing climate change should lead to larger horns. Seasonal values of PDO were highly correlated. Horn growth increased with PDO in spring or summer at low density, but was weak at high density regardless of PDO. The interaction between population density and PDO in spring or summer accounted for a similar proportion of the observed annual variation in horn growth (32% or 37%) as did the additive effects of spring temperature and density (34%). When environmental conditions deteriorated, males allocated relatively more resources to summer mass gain than to horn growth, suggesting a conservative strategy favoring maintenance of condition over allocation to secondary sexual characters. Population density explained 27% of the variation in horn length, while evolutionary effects explained 9% of the variance. Thus, our study underlines the importance of both evolution and phenotypic plasticity on the development of a secondary sexual trait.

雄性第二性征（secondary sexual characters）如鹿角或牛角的发育兼具重要的生物学与社会经济意义：在诸多物种中，这类性状通过性选择对雄性适合度产生正向影响，却又因战利品狩猎（trophy hunting）带来负面效应。环境条件与选择性狩猎均可能影响角的生长，但二者的相对重要性尚未得到明确探究。我们首先针对42年间的加拿大盘羊（Ovis canadensis，俗称大角羊）雄性个体，探究了大尺度气候指数太平洋年代际振荡（Pacific Decadal Oscillation, PDO）、局地天气状况以及种群密度，如何影响其从出生至3岁期间的绝对与相对年度角生长量。随后我们分析了以战利品狩猎为主导的演化过程与环境条件，对3岁雄性加拿大盘羊角长的相对影响。研究发现，角生长量受低种群密度与温暖春季气温的正向影响，这意味着持续的气候变化或将导致角的尺寸增大。春季与夏季的PDO数值呈现高度相关性。在种群密度较低时，角生长量随春季或夏季PDO升高而增加；但在种群密度较高时，无论PDO如何变化，角生长量的响应均较弱。春季或夏季的种群密度与PDO的交互作用，对角生长年度变异的解释度（分别为32%与37%），与春季气温和种群密度的加性效应（34%）相当。当环境条件恶化时，雄性个体会将更多资源分配至夏季体重增长而非角生长，这表明其采取了保守策略：优先维持身体状态，而非将资源投入第二性征的发育。种群密度可解释27%的角长变异，而演化效应仅解释了9%的变异。因此，本研究凸显了演化与表型可塑性（phenotypic plasticity）对第二性征发育的共同重要性。